1. Field of the Invention
The present invention relates to an optical attenuator that uses an element of a micro-electro-mechanical system (MEMS) device, and more particularly to an MEMS variable optical attenuator capable of amplifying a displacement distance of an optical shutter so that the displacement distance is compatible with a large mode field diameter (MFD) of an optical signal transmitting end or an optical signal receiving end of an optical fiber.
2. Description of the Related Art
An optical attenuator is an optical component for use in optical telecommunication networks. The optical attenuator includes a pair of optical waveguides having an optical signal transmitting end and an optical signal receiving end, respectively, and attenuates an optical power of an optical beam passing out the transmitting end of the optical waveguide and entering the receiving end of the optical waveguide by causing insertion loss of the optical beam.
Generally, optical power levels are regulated over wide ranges based on a configuration of optical telecommunication systems. For example, the optical power levels are determined by an optical transmission loss typically varied based on a length of an optical transmission line, the number of connection points of optical fibers, and the number and performance of optical components such as optical couplers coupled to the optical transmission line. An optical attenuator is needed in optical telecommunication networks to reduce an optical power when an optical signal with a excessive power level greater than an allowed power level is received by an optical signal receiver. The optical attenuator further may be used in evaluating, adjusting and correcting telecommunication equipments and optical measurement equipments.
Such optical attenuators are classified into two types, a fixed optical attenuator for reducing an optical power by a fixed amount of attenuation and a variable optical attenuator capable of attenuating an optical power of incident light beams by a varied amount of attenuation based on user's requirements. Such optical attenuators are required to be produced at low cost with high reliability and small size.
To satisfy such requirements, an optical attenuator that uses an element of an MEMS device has been suggested. Such MEMS optical attenuator is realized by forming a microstructure acting as an actuator on a substrate such as silicon by using a thin film processing technology. Generally, an MEMS actuator is driven to move by a driving force caused by thermal expansion or an electrostatic force. As the MEMS actuator moves, an optical shutter coupled to the MEMS actuator is displaced so as to be inserted into a gap between two optical waveguides, thereby partially intercepting optical beams traveling from an optical signal transmitting end (or an exit end) of the optical waveguide such as an optical fiber to an optical signal receiving end (or an incident end) of the optical waveguide.
FIGS. 1A and 1B illustrate a perspective view and a plan view, respectively, of a conventional variable optical attenuator using an actuator driven by an electrostatic force.
Referring to FIGS. 1A and 1B, an MEMS variable optical attenuator includes a substrate having a pair of optical waveguides 19a, 19b provided thereon, wherein one waveguide has an optical signal transmitting end and the other has an optical signal receiving end, an electrostatic actuator comprised of driving electrodes 12a, 12b, a ground electrode 14, a spring 15 and a movable mass 16, and an optical shutter 17 connected to the movable mass 16 of the electrostatic actuator.
The driving electrodes 12a, 12b and the ground electrode 14 are supported by an oxide layer called an “anchor” and formed on the substrate 11, and thereby fixed to the substrate 11. The movable mass 16 is connected to the ground electrode 14 via the spring 15 and has a comb shape. The driving electrodes 12a, 12b have respective extended portions 13a, 13b, each with a comb shape. The comb of each of the extended portions 13a, 13b is interdigitated with the comb of the movable mass 16.
When driving signals are applied to the driving electrodes 12a, 12b so as to generate a potential difference between the driving electrodes 12a, 12b and the ground electrode 14, an electrostatic force arises between the interdigitated combs of movable mass 16 and extended portions 13a, 13b, thereby causing the movable mass 16 to move. As the movable mass 16 moves, the optical shutter 17 is inserted into a gap defined by the optical signal transmitting end 19a and the optical signal receiving end 19b so as to partially intercept optical beams incident onto the optical shutter 17.
Advantageously, optical waveguides are optical fibers. To improve optical performance of the optical fibers, an optical collimator can be used. The optical collimator enlarges a mode field diameter of the optical fiber, thereby reducing alignment loss of optical beams, amount of variation of wavelength dependence loss (WDL) and polarization dependence loss (PDL) of light beams, reflection loss and initial insertion loss of light beams. As a result, it is possible to achieve a superior optical performance of the optical fiber.
However, even though the optical collimator has such advantages as described above, it cannot be adopted in a conventional MEMS variable optical attenuator due to its large mode field diameter (MFD). The conventional MEMS variable optical attenuator is provided with an actuator having a driving stroke of about 10 μm which is compatible with a MFD of a typical optical fiber. However, in the case of using an optical collimator, a MFD of the optical fiber increases to 100 μm, or to 200-300 μm under certain circumstances, so that it is difficult to achieve an adequate attenuation level of the incident light beams by using the conventional MEMS actuator having a short driving stroke.
To solve the above problem, it is necessary to lengthen the actuator's driving stroke so that a displacement distance of an optical shutter increases, but there is a limit to lengthening a driving stroke of an actuator because an MEMS variable optical attenuator is implemented in a very small sized chip. In a conventional MEMS variable optical attenuator, a driving stroke of an actuator is limited by a gap “d” defined by two facing combs, a comb of the movable mass 16 and a comb of the extended portions 13a, 13b of the driving electrodes 12a, 12b. Accordingly, if the driving stroke of the actuator is lengthened to be compatible with the MFD of the optical collimator only by using the gap “d”, it cannot satisfy the need for a small sized MEMS optical variable attenuator.
Accordingly, to realize an MEMS variable optical attenuator having an excellent optical performance and a small size, it is necessary to modify a structure of an MEMS actuator so that a driving stroke of the MEMS actuator can be amplified to be compatible with a large MFD of an optical collimator.